U.S. patent application number 14/341239 was filed with the patent office on 2014-11-13 for vcsel pumped fiber optic gain systems.
This patent application is currently assigned to PRINCETON OPTRONICS INC.. The applicant listed for this patent is Princeton Optronics Inc.. Invention is credited to Chuni Lal Ghosh, Jean-Francois Seurin, Qing Wang, Laurence Watkins.
Application Number | 20140333995 14/341239 |
Document ID | / |
Family ID | 50184861 |
Filed Date | 2014-11-13 |
United States Patent
Application |
20140333995 |
Kind Code |
A1 |
Seurin; Jean-Francois ; et
al. |
November 13, 2014 |
VCSEL Pumped Fiber Optic Gain Systems
Abstract
Optical pump modules comprising VCSEL and VCSEL array devices
provide high optical power for configuring fiber optic gain systems
such as fiber laser and fiber amplifier particularly suited for
high power operation. Pump modules may be constructed using two
reflector or three reflector VCSEL devices optionally integrated
with microlens arrays and other optical components, to couple high
power pump beams to an optical fiber output port, particularly
suited to couple light to an inner cladding of a double-clad fiber
suitable for a high power gain element. Multiple-pumps may be
combined to increase pump power in a modular fashion without
significant distortion to signal, particularly for short pulse
operation. The pump modules may be operated in CW, QCW and pulse
modes to configure fiber lasers and amplifiers using single end,
dual end, and regenerative optical pumping modes.
Inventors: |
Seurin; Jean-Francois;
(Princeton Junction, NJ) ; Wang; Qing;
(Plainsboro, NJ) ; Watkins; Laurence; (Pennington,
NJ) ; Ghosh; Chuni Lal; (West Windsor, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Princeton Optronics Inc. |
Mercerville |
NJ |
US |
|
|
Assignee: |
PRINCETON OPTRONICS INC.
Mercerville
NJ
|
Family ID: |
50184861 |
Appl. No.: |
14/341239 |
Filed: |
July 25, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13783172 |
Mar 1, 2013 |
8824519 |
|
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14341239 |
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Current U.S.
Class: |
359/341.3 |
Current CPC
Class: |
H01S 5/141 20130101;
H01S 5/02208 20130101; G02B 6/4206 20130101; H01S 3/094053
20130101; H01S 5/183 20130101; H01S 3/094007 20130101; H01S 3/09415
20130101; H01S 3/235 20130101; H01S 5/18305 20130101; H01S 3/06754
20130101; H01S 5/423 20130101; H01S 5/50 20130101; H01S 3/11
20130101; H01S 3/091 20130101; H01S 5/005 20130101; H01S 5/4012
20130101; H01S 3/1068 20130101; H01S 3/10 20130101; H01S 5/02284
20130101; H01S 3/0675 20130101; H01S 5/026 20130101 |
Class at
Publication: |
359/341.3 |
International
Class: |
H01S 5/50 20060101
H01S005/50; H01S 5/183 20060101 H01S005/183 |
Claims
1. A regenerative fiber optic amplifier system comprising: a fiber
optic pump module including a plurality of VCSEL emitters and an
optical fiber to output a pump beam generated in said optical pump
module; a first reflector having a reflectivity significantly
higher than the reflectivity of a second reflector; a fiber gain
element having a core doped with a plurality of optically active
ions, wherein a first end of the fiber gain element is connected to
the optical fiber via the first reflector so as to couple the pump
beam to the fiber gain element; an acousto-optic switch including
at least three ports, wherein one port of the acousto-optic switch
is connected to a second end of the fiber gain element, and a
second port of the acousto-optic switch is connected to the second
reflector, so as to form a regenerative resonant cavity between the
first and second reflectors enclosing the fiber gain element via
the acousto-optic switch; a circulator having at least three ports
that are independent of each other, wherein a first output port of
the circulator is connected to the second port of the acousto-optic
switch; an optical signal source connected to an input port of the
circulator to transmit a signal pulse to the regenerative resonant
cavity through the first output port of the circulator and the
acousto-optic switch, wherein upon closing the acousto-optic switch
in synchronization with pulse operation of the pump module, the
pump beam recirculates until the signal in the regenerative
resonant cavity amplifies to a saturation limit, and subsequently
upon opening the acousto-optic switch, an amplified signal pulse is
transmitted from the gain element to a third port of the circulator
via the acousto-optic switch.
2. The fiber optic amplifier system as in claim 1, wherein the
optical pump module further includes: a thermal mount, wherein the
plurality of VCSEL emitters are disposed in thermal contact with
the thermal mount; one or more optical components, said optical
components disposed at a predetermined distance following said
plurality of VCSEL emitters to collimate and combine respective
optical beams emitted from the plurality of VCSEL emitters to have
a substantially uniform intensity in the pump beam; an optical
coupling device disposed at a pre-determined distance following the
one or more optical components; an optical alignment housing
positioned at a pre-determined distance following said optical
coupling device to support and coaxially align an input end of the
optical fiber with the optical coupling device and the pump beam
for coupling a maximum intensity of the pump beam to the input end
of the optical fiber to provide the output pump beam at an output
end of the optical fiber.
3. The fiber optic amplifier system as in claim 2, wherein one or
more optical components is one selected from a group consisting of
lens, one or more microlenses substantially aligned with a
corresponding one or more of the plurality of VCSEL emitters, and a
combination thereof.
4. The fiber optic amplifier system as in claim 2, wherein the
optical coupling device includes a lens or a taper optical
component.
5. The fiber optic amplifier system as in claim 2 further
including: an external housing, said housing having a thermally
conducting base plate in thermal contact with the plurality of
VCSEL emitters, and wherein the plurality of VCSEL emitters, the
one or more optical components, the optical coupling device, and
the fiber alignment housing are secured coaxially on the base
plate; and an external elevated cover, said cover to enclose the
fiber optic pump module on the base plate, wherein said cover
further includes a fiber alignment guide and a bend radius limiter
on one end to support and secure the optical fiber coming out of
the external housing, and wherein said external housing is
adaptable for an external cooling device placed in thermal contact
with the base plate using a circulating fluid that is one selected
from the group consisting of a gas, a liquid and a combination
thereof.
6. The fiber optic amplifier system as in claim 1, wherein the
plurality of VCSEL emitters in the optical pump module are one
selected from the group consisting of a two reflector VCSEL, a
three reflector VCSEL and a combination thereof, and wherein the
plurality of VCSEL emitters are arranged in linear or two
dimensional planar arrays configured to emit collectively.
7. The fiber optic amplifier system as in claim 1, wherein the
optical fiber is a double-clad fiber and the pump beam is coupled
to the core, an inner cladding, or a combination thereof.
8. The fiber optic pump module as in claim 1, wherein the optical
fiber is a single-clad fiber, wherein the pump beam from the
plurality of VCSEL emitters is coupled to the core of the
fiber.
9. The fiber optic amplifier system as in claim 1, wherein the
fiber gain element comprises a single-clad fiber and the pump beam
is coupled to the core of the gain element fiber.
10. The fiber optic amplifier system as in claim 1, wherein the
fiber gain element comprises a double-clad fiber and the pump beam
is coupled to the gain element fiber in the core, an inner
cladding, or a combination thereof.
11. The fiber optic amplifier system as in claim 1 further
including one or more isolators, wherein at least one isolator
precedes, and at least one isolator follows the fiber gain
element.
12. The fiber amplifier as in claim 11, wherein the one or more
isolators are constructed using a double-clad fiber having a core
and an inner cladding matched respectively, to the core and inner
cladding of the fiber gain element.
13. The fiber optic amplifier system as in claim 1, wherein the
first and the second reflectors are fiber Bragg gratings having
respectively, a high reflectivity core and a low reflectivity
core.
14. The fiber optic amplifier system as in claim 13, wherein the
first and second reflectors are constructed in the core region of a
double-clad fiber.
15. The fiber optic amplifier system as in claim 1 further
including additional one or more optical pump module, wherein the
additional one or more pump module are constructed using a
plurality of VCSEL emitters that are arranged in a linear or
two-dimensional array.
16. The fiber optic amplifier system as in claim 15, wherein a
plurality of pump beams generated from the pump module and the
additional one or more pump module are arranged around the input
end of the first reflector to combine the plurality of pump beams
using a single optical component.
17. The fiber optic amplifier system as in claim 15, wherein a
plurality of pump beams generated from the pump module and the
additional one or more pump module is combined in a double-clad
fiber combiner, said combiner having one output fiber and multiple
input fibers, wherein at least one input fiber of said combiner has
a core connected to the core of said output fiber, so as to combine
the plurality of pump beams propagating in the inner cladding of
each input fiber to the inner cladding and core of the output fiber
to facilitate coupling the plurality of pump beams to the first
reflector.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a `Divisional` of the U.S. patent
application Ser. No. 13/783,172 filed on Mar. 1, 2013, by Seurin et
al., the contents of which is being `incorporated by reference` in
its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] The present invention relates to fiber optic gain systems
and in particular to fiber optic lasers and amplifiers including
Vertical Cavity Surface Emitting Laser (VCSEL) optical pumps.
[0004] 2. Background Art
[0005] Fiber optic lasers and amplifiers (hereinafter referred as
fiber lasers and fiber amplifiers, respectively) are well known in
the art for their excellent conversion efficiency, beam quality,
small volume, light weight, and low cost. Primarily, fiber lasers
and fiber amplifiers have an optical gain medium, typically
comprising an optical fiber doped with optically active ions in a
core region surrounded by a cladding region. For example, the core
may be doped with selective rare-earth ions which upon absorbing
optical power from an intense light source such as a high power
optical pump, are excited to a higher energy state and when the
excited ions return to their ground state they provide optical gain
or amplification. The light generated in the doped fiber is
reflected back into the cavity for resonant amplification.
[0006] Primary application of low power lasers and amplifiers for
example, in 10-100 mW (milli-Watt) range is for amplifying optical
signals in telecommunications. For other industrial and military
applications of lasers that are very diverse, for example, cutting,
welding, marking, etc. high output power is required. The output
can be multimode or single mode and results in very high power
densities in a focused beam. In addition the fiber laser or
amplifier output may be generated to operate in continuous mode or
pulsed with various pulse widths and repetition rates. In a recent
non-patent literature publication entitled "110 W Fiber Laser",
presented in a Conference on Laser and Electro Optics (paper
CPD11-1, at CLEO '99, 23-28 May 1999, Baltimore, Md.), Dominic et
al., described a high power fiber optic laser using an Yb
(Ytterbium) doped double-clad optical fiber specially designed to
have a rectangular cross section cladding and a single mode core.
In the configuration described therein, the double-clad fiber is
pumped at both ends by polarization combining two laser diode bar
packages, each package having an output power of 45 W, to achieve a
total 180 W of optical pump power.
[0007] Most prior art fiber lasers and amplifiers focused on
optimizing the fiber design and in particular, design of the fiber
core, so as to couple pump radiation optimally. Advancements in
core region designs made significant improvement in coupling pump
light using different types of high power optical sources including
edge emitting laser diode bars, Raman pumps, Erbium Doped Fiber
Amplifier (EDFA) pumps and Vertical Cavity Surface Emitting Laser
(VCSEL) particularly single VCSEL or a linear array of VCSEL, etc.
Significant progress in attaining high power in fiber lasers and
amplifiers may be attributed to advancements in new types of
optical fibers particularly, a double-clad optical fiber. In a
double-clad fiber, an inner cladding is used as a pump cavity,
whereas the outer cavity prevents the pump radiation from leaking
out. More specifically, pump radiation is coupled into the inner
cladding and as it propagates down the fiber it propagates in and
out of the core region but stays confined within the inner cladding
at its boundary with the outer cladding.
[0008] In several prior art fiber lasers, inner cladding having in
different shapes are constructed for optimally coupling the pump
radiation to the core region. For example, in the U.S. Pat. No.
4,815,079 issued to Snitzer et al. on Mar. 21, 1989, a fiber laser
and amplifier is constructed from a double clad fiber having a
single mode core placed off-centered with respect to a multi-mode
inner cladding and an outer cladding. The pump radiation is
launched in the inner cladding layer. In addition, the fiber is
configured to be slightly bent such that modes that would not
ordinarily couple with the single mode core would couple pump power
to the core to ensure efficient coupling of the pump power.
[0009] A different configuration for efficient coupling of pump
power to a double-clad fiber is described in the U.S. Pat. Nos.
5,949,941, and 5,966,491 both issued to DiGiovanni on Sep. 7, 1999
and Oct. 12, 1999, respectively. In this design, an inner cladding
region includes a stress inducing region around the core which is
asymmetric, followed by a second cladding layer. The stress
inducing region is for generating refractive index modulation to
increase the pump radiation mode diversity to increase pump
radiation propagation through the core region. A similar design is
also described in another U.S. Pat. No. 6,477,307 issued to Tankala
et al. on Nov. 5, 2002 having a inner cladding region with multiple
sections that are designed to increase the amount of propagation of
the pump radiation in the core region.
[0010] Output power in a fiber laser or amplifier is determined by
input power from the pump source(s) as well as the proportion of
the wavelength band which aligns with the absorption band linewidth
of the active ions in the core region. In U.S. Pat. No. 7,593,435
issued to Gapontsev et al. on Sep. 9, 2009, a fiber laser capable
of delivering 20 kW output power in a single mode beam is
described. However this is a very complex arrangement where
multiple single mode Er doped fiber amplifiers using Raman pumps,
are used as pump source. The multiple fiber amplifiers are combined
in single mode to multimode fiber combiners to generate a high
power pump source. The narrow wavelength band closer to the
emission wavelength, aligns accurately with the doped core fiber
absorption line resulting in very efficient optical pumping due to
high power, narrow linewidth and low photon defect.
[0011] As an alternative, edge emitting semiconductor lasers, a
linear array of plurality of such lasers (or a laser bar), and
extended emitter laser diodes (or arrays) emitting at wavelengths
corresponding to absorption wavelength of doped ions in the fiber
core are conventionally used as pump sources. One such pump
configuration is described in U.S. Pat. No. 4,829,529 issued to
Kafka on May 9, 1989. More specifically, a single mode fiber is
embedded in a multi-mode cladding and an outer cladding layer to
form a pump cavity such that the light from the pump source is
confined within the pump cavity, by total internal reflection. The
pump radiation to the core is coupled along the length of the
fiber.
[0012] Another variation of pumping an inner cladding layer is
described in the U.S. Pat. No. 6,801,550 issued to Snell et al. on
Oct. 5, 2004. In this device a cladding layer in a double cladding
fiber includes V-grooves to couple pump radiation efficiently to
the core using multiple emitters placed along the length of the
doped fiber. However, one disadvantage of this configuration is
that the V-groove is designed integral to the cladding layer and
works well for a specific pump wavelength.
[0013] While pumps using edge emitter lasers are useful for pumping
doped core of fiber lasers and amplifiers, edge emitter lasers have
a relatively broad linewidth and all the light emitted by the pump
laser does not contribute towards exciting the doping ions which
absorb only in a narrow wavelength band. Furthermore, edge emitter
laser wavelength varies considerably with operating temperature
resulting in misalignment in the pump wavelength and the absorption
lines of the doping ions. Therefore, each edge emitting laser in a
bar requires a temperature controller mechanism to stabilize
respective operational wavelength.
[0014] In recent years, optical pumps for solid state lasers have
been configured using VCSEL devices. VCSELs have very narrow
linewidth and in addition their wavelength varies much less with
temperature and drive current. The VCSEL chip can be configured
with arrays of VCSEL devices in two dimensions which results in
very high output power from a single array. In the U.S. Pat. No.
6,888,871 issued to Zhang et al. on May 3, 2005, an optical pump
using VCSEL array is described for pumping a solid state laser.
VCSELs and arrays of VCSELs are very compact and may be easily
integrated with other devices for providing additional optical and
control functions to the pump.
[0015] In other prior art patent publication, for example, in the
U.S. Pat. No. 7,295,375 issued to Jacobowitz et al. on Nov. 13,
2007, VCSEL arrays in combination with micro lenses and optical
interconnects are described. A compact packaging for VCSEL arrays
to a fiber cable is described in the U.S. Pat. No. 6,984,076 issued
to Walker Jr. et al. on Jan. 10, 2006. However, each individual
VCSEL is wire bonded to a contact pad and to the external
connectors of the packaging. One major disadvantage of this
approach is that for high optical power application, the wire
bonding is prone to failure, thereby imposing a practical
limitation on output power that can be obtained from such
pumps.
SUMMARY OF THE INVENTION
[0016] In this invention, an optical pump comprising a high output
power optical module including a VCSEL or a VCSEL array device
coupled to an optical fiber particularly suited to configure high
power fiber optic gain systems is provided. The optical module may
be configured using different combinations of VCSEL arrays and
optical components for application requiring high power output.
[0017] One important aspect of the high output power module
provided in this invention is a three reflector VCSEL device for
better wavelength stability of the pump modules in high power fiber
gain systems such as, a fiber laser and an amplifier. The optical
module together with fiber components are configured to provide a
fiber laser and an amplifier. Multi-stage fiber optic systems for
example, fiber laser, single or multi-stage amplifiers and other
combinations thereof may be configured in a modular fashion for
extending the power output requirements for different
applications.
[0018] In one embodiment of the invention a high power optical pump
is provided. The high power optical pump comprises one or more
VCSEL or VCSEL arrays. In one variation of the invention VCSELs or
VCSEL arrays are packaged with one or more optical components for
beam shaping and focusing. The output of the VCSEL or VCSEL array
is coupled to a fiber pigtail using a micro-lens array, a single
lens or a combination thereof. In another embodiment of the
invention, selection of one or more VCSELs or VCSEL array(s) and
the array(s) dimension is determined by required output power.
Advantageously, the optical module may be constructed from
conventional two reflector VCSEL devices or from high power
three-reflector VCSEL devices constructed according to this
invention.
[0019] In another embodiment, a fiber laser is provided by coupling
light from a high power optical pump module comprising one or more
VCSELs or VCSEL arrays. In practice, light from the pump module is
coupled to a doped silica fiber gain medium. The doped fiber gain
medium may comprise a double-clad silica fiber where pump radiation
is confined within an inner cladding layer. The gain medium is
placed between two reflectors, one reflector preferably having
reflectivity smaller than the other, to direct the output laser
light. The light from the VCSEL pump module may be coupled directly
to the gain medium using a focusing lens, tapered optics, or via
optical components such as a combiner, a circulator, etc.
[0020] In a variant configuration, the doped silica fiber gain
medium is pumped from both ends by providing separate optical pump
modules at the two ends of the gain medium. Additional optical
components such as an acousto-optic switch may be provided in some
applications. The fiber laser may be operated in continuous wave
(CW) mode, quasi continuous wave (QCW) mode or pulse (P) mode.
Furthermore, fiber laser may be operated for short pulses by
operating the pump module using a current source operated in pulse
mode. The pulse width of the laser may further be reduced by
operating the laser cavity in Q-switching mode.
[0021] In yet another embodiment a fiber amplifier is provided by
coupling light from a high power optical pump module comprising one
or more VCSELs or VCSEL arrays. Light from the pump module is
combined with the signal to be amplified in the core of a doped
silica fiber gain medium, having a single-clad or, preferably a
double-clad silica fiber where pump radiation is confined within an
inner cladding layer. The signal and pump are coupled using a
conventional combining device such as, an optical combiner, a
multiplexer or a polarization combiner, etc. or a combining device
specially designed to operate with double-clad fiber. The amplifier
so configured may be operated to amplify any optical signal
including a short pulse (seed pulse). In one embodiment a short
pulse regenerative amplifier is provided. A multi-stage fiber
amplifier may be configured by modularly connecting several
amplifiers for enhancing output power.
[0022] In a different embodiment, a fiber laser may be cascaded
with a fiber amplifier in order to boost laser output power. The
combination may provide a desirable level of laser power without
increasing the pump input power to operate the fiber laser. In an
alternative configuration of fiber laser and amplifier, output from
several pump modules may be combined to enhance input optical pump
power to a doped fiber gain medium in order to obtain higher output
power. In yet another embodiment, pump modules may be applied to a
gain medium from one end or from both ends.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] A broad framework of the invention is described using
different embodiments described in the specification which will be
better understood in conjunction with the drawing figures in
which:
[0024] FIG. 1 shows a schematic representation of a basic fiber
gain system suitable to practice the invention;
[0025] FIG. 2 shows schematic representations of different types of
top emitting VCSEL devices, a two reflector VCSEL device (2a), and
extended cavity VCSEL device including, an integrated third
reflector (2b), third reflector externally bonded to the device
(2c), and external third reflector (2d), respectively;
[0026] FIG. 3 shows schematic representations of different types of
bottom emitting VCSEL devices, a two reflector VCSEL device (3a),
and extended cavity VCSEL device including, an integrated third
reflector (3b), third reflector externally bonded to the device
(3c), and external third reflector (3d), respectively;
[0027] FIG. 4 is a schematic representation of two dimensional
VCSEL array configurations, two reflector or extended cavity VCSELs
(a) and, extended cavity VCSELs with external reflector (b);
[0028] FIG. 5 is a schematic drawing showing placement of a
microlens array with the VCSEL array for obtaining a uniform
emission pattern collectively;
[0029] FIG. 6 depicts coupling of pump radiation to inner cladding
of a fiber, direct coupling (a), lens coupling (b) and, an optical
taper coupling (c), respectively;
[0030] FIG. 7 depicts a VCSEL array optical pump module including
an optical fiber output port;
[0031] FIG. 8 is a schematic representation of a fiber laser
configured using a VCSEL pump module, inset shows a double-clad
fiber cross section;
[0032] FIG. 9 is a schematic representation of a fiber laser
configured in dual pumping mode using VCSEL pump modules;
[0033] FIG. 10 is a schematic representation of a fiber laser
configured in a Q-switch mode for obtaining short laser pulse;
[0034] FIG. 11 shows representative performance characteristics of
fiber laser configured using VCSEL pump modules;
[0035] FIG. 12 shows representative performance characteristics of
fiber laser configured using VCSEL pump modules;
[0036] FIG. 13 shows representative performance characteristics of
fiber laser configured using VCSEL pump modules;
[0037] FIG. 14 is a schematic representation of a fiber amplifier
configured using VCSEL pump module;
[0038] FIG. 15 is a schematic representation of a laser amplifier
configured to operate in a regenerative mode; and
[0039] FIG. 16 is a schematic representation of a fiber laser or an
amplifier configured with multiple VCSEL pump modules coupled into
a single fiber gain element.
DETAILED DESCRIPTION
[0040] A broad framework of the principles will be presented by
describing various aspects of this invention in exemplary
embodiments shown in different drawing figures. For clarity and
ease of description, each embodiment includes only a few aspects.
However, different aspects from different embodiments may be
combined or used separately, to put the invention to practice. Many
different combinations and sub-combinations of the representative
embodiments within the broad framework of this invention, that may
be apparent to those skilled in the art but not explicitly shown or
described, should not be construed as precluded.
[0041] FIG. 1 shows an exemplary fiber optic gain system 100 to
explain different aspects of the invention that may be configured
to practice the invention in different modes. The fiber optic
system is represented in terms of basic building blocks to
illustrate the broad principles and should not be construed as
limiting. Details of each block will be elaborated later. More
specifically, the fiber optic gain system comprises an optical pump
module 101 including an output fiber 102 (also referred
synonymously as a fiber pigtail). The pump module further includes
an optical emitter 111 and a focusing device 112 to focus the light
from the emitter to the fiber pigtail 102. The pump module will be
described later in detail. Optical output from the pigtail 102 of
the pump module is spliced, preferably by fusion splicing, to a
fiber pigtail 104 of a gain element section 105. In an alternative
arrangement, a low-loss connector 103 may also be used for light
coupling from the pump module to the gain element section.
[0042] The gain element 105 forms the active part of the gain
system. The gain element is a fiber section where the core is doped
with active ions that include but are not limited to, Ytterbium,
Erbium, Neodymium, Thulium, Praseodymium, etc. and are well known
dopants for fiber gain elements. The gain element may include a
single-clad fiber including only one layer of cladding, or
preferably a double-clad fiber including an inner cladding to
couple the pump radiation and an outer cladding to confine the pump
radiation to the core and inner cladding of the fiber. It is
important to note that the pump module pigtail may also be a double
clad fiber that is matched to the gain element fiber pigtail to
minimize coupling losses. In some instances, a small piece of input
fiber is used, such that one end matches the gain element and the
other end matches the pump module pigtails, respectively. These and
other technical aspects for achieving low loss coupling are well
known in the art and will not be described in more detail.
[0043] The pump radiation excites the active ions in the core to a
higher electronic state for lasing or amplification action. The
output end 106 of the gain element is connected to an output fiber
107 preferably by a fusion splice or a low-loss connector similar
to 103 shown at the input end. The output of the gain element may
generate a single mode or a multi-mode diverging beam which is
collimated into an output beam 109, using collimating optics 108
which may include but is not limited to, a single lens, a
combination of lenses, or a combination of lenses and light guiding
optics, etc. For better stabilization of the output beam, an
isolator 110 may optionally be included after the gain section to
reduce the reflected light from the object(s) illuminated by the
output beam. Additional optical elements such as, reflectors,
gratings, couplers, circulators, acousto-optic switches, etc. may
be appropriately positioned on either side of the fiber gain
section to configure the gain element to be operated as a laser or
an amplifier, and will be described in more detail later.
Optical Pump Module:
[0044] In one aspect, the invention provides a high power optical
pump module comprising single high output power VCSEL devices or
multiple VCSEL devices arranged in two-dimensional arrays. Optical
illuminators constructed by arrays of VCSEL have been disclosed in
a U.S. patent application Ser. No. 13/541,906, filed on Jul. 5,
2012, that issued as the U.S. Pat. No. 8,675,706 to Seurin et al.
on Mar. 18, 2014, and assigned to Assignee of this application.
Contents of that application co-authored by some of the inventors
of this application and co-owned by Princeton Optronics Inc.,
Mercerville, N.J., is being incorporated by reference in its
entirety. The VCSEL devices disclosed in the above referenced
application are conventional VCSEL devices configured using
two-reflectors placed on either side of an active layer. In FIG. 2,
several VCSEL device structures including, a conventional device
are shown. Identical elements having similar functions in the
devices shown in FIG. 2 are labeled with same reference numerals
and the same description applies.
[0045] More specifically, a two reflector self lasing VCSEL devices
shown in FIG. 2a, is constructed on a substrate 201 including a
first electrical contact layer 202 (also a first electrical
terminal). A light emitting region 204 which is a semiconductor
gain medium is disposed between two reflectors 203 and 206. The
reflectors can be of various types such as distributed Bragg
gratings, including dielectric or semiconductor, gratings including
semiconductor, dielectric or metal, or reflecting metal. A second
electrical contact layer 207 (also a second terminal) is formed on
a surface opposite to that of the substrate contact 202. A current
confinement aperture 205 controls the flow of drive current to the
light emitting region and also determines the shape of emission
beam, as has been described in the co-authored and co-owned pending
U.S. patent application Ser. No. 13/337,098 by Seurin et al., filed
on Dec. 24, 2011. A transparent window 208 aligned with the current
confinement aperture is provided on the VCSEL surface for the laser
output 209 to be emitted in a vertical direction (with respect to
the plane of the substrate in this illustration).
[0046] The conventions adopted here only for the purpose of
illustration and ease of description, should not construed to be
limiting. For the purpose of discussion following convention would
be adopted--reference to a `top` and `bottom` ends or a `top` and
`bottom` electrical contacts of the device is in reference to an
emission surface. Accordingly, the emission end and the electrical
contact on the top end of a device would be referred as the top end
and the top contact, respectively. The non-emission end of the
device and the electrical contact to the non-emission end would be
referred as the bottom end and the bottom contact, respectively in
the exemplary embodiments throughout, unless stated otherwise.
Accordingly, the top emission device shown in FIG. 2a has the
emission surface 208 located opposite to the substrate end and the
VCSEL emission 209 is from a window above the light emitting region
204.
[0047] A prior art device shown in FIG. 2a may be used to construct
a pump module particularly for low power applications for example,
to pump a solid state lasers as has been disclosed in the U.S.
patent application Ser. No. 13/541,906, filed on Jul. 5, 2012, that
issued as the U.S. Pat. No. 8,675,706 to Seurin et al. on Mar. 18,
2014, and assigned to Assignee of this application. A high power
pump module constructed using plurality of conventional two
reflector device arranged in arrays may be constructed to optically
pump fiber gain systems in some applications, and will be described
shortly. However, better pump modules having far more desirable
output characteristics including but not limited to, higher output
power in a single mode, better wavelength stability (with
temperature over longer time operation), etc., have been developed
using extended cavity VCSELs constructed according to this
invention. Exemplary extended cavity VCSEL shown in FIGS. 2b-2d
devices have a basic structure very similar to the prior art device
shown in FIG. 2a except that it includes an additional third
reflector to achieve some of the desirable characteristics
mentioned earlier.
[0048] More specifically, FIG. 2b shows a top emitting VCSEL device
having three reflectors. The device constructed on a substrate 201
has a light emitting region 204 disposed between a first reflector
203 and a second reflector 206. The reflector 206 in this device is
made to be partially reflecting and a third reflector 210 is
fabricated on the bottom surface of the substrate opposite to the
surface with the light emitting region. The reflectors 206 and 210
when designed with specific phase relationship, results in high
reflectivity such that the combination, together with the reflector
203 provides desired lasing operation. Electrical contact to the
substrate is made through the third reflector 210. Similar to the
prior art device shown in FIG. 2a, the output 209 is still
transmitted out of the reflector 203 in this exemplary
configuration.
[0049] In a variant exemplary embodiment of an extended cavity
VCSEL device shown in FIG. 2c, a top emitting VCSEL device is
constructed on a substrate 201 has a light emitting region 204
disposed between reflectors 203 and 206, respectively. Unlike the
device described in reference with FIG. 2a, the reflector 203 in
this embodiment forms the middle reflector and is made partially
reflective to include a third reflector 210 to configure an
extended cavity device. The substrate 201 has a selective
antireflection coating 211 applied to the surface opposite to the
light emitting region 204. More specifically, the antireflection
coating is applied in regions where the optical beam traverses
between the reflectors 203 and 210. A metallized contact 202 is
formed at the bottom in the regions not covered with the
antireflection coating.
[0050] An extended cavity is formed between the reflector 206 and
the third reflector 210 which is deposited on a transparent
substrate 213 and has an antireflection coating 212 applied to one
surface while the opposite surface is coated with a metal layer 215
to facilitate bonding to a heat sink. The surface of the
transparent substrate including the antireflection coating has
metallization 214 applied to the areas outside the region where the
optical beam traverses. The third reflector 210 is located below
the bottom surface of the VCSEL device at a design distance,
determined by the thicknesses of the VCSEL and transparent
substrates 201 and 213, such that the combined phase matched
reflection from the reflectors 203 and 210 provide a high
reflectivity to produce laser action having the desired output
characteristics including, high output power in single mode, better
wavelength stability and uniform beam shape. The third reflector on
the transparent substrate is attached to the substrate 201 at the
surface having the antireflection coating 211, using a solder 216
to form a monolithic module. The resonant laser action occurs in
the cavity formed by the three reflectors 210, 203 and 206. The
laser output light 209 emits from the reflector 206.
[0051] In a different embodiment of the invention an extended
cavity VCSEL may also be configured using an external reflector as
shown in FIG. 2d. In this embodiment the basic device is very
similar to a prior art device shown in FIG. 2a. More specifically,
the VCSEL device comprises a substrate 201 on which a light
emitting region 204 is disposed between two reflectors 203 and 206,
respectively. Reflectivity of the reflector 206 is substantially
reduced such that an external third reflector 210 having a
pre-determined reflectivity is positioned at a pre-determined
height above the reflector 206. In particular, the third reflector
is placed above the VCSEL emission window 208 such that the
combined phase matched reflectivity of the reflectors 206 and 210
is high enough to provide laser output 219 in the cavity formed by
reflectors 203, 206 and 210. The output 209 of the laser is emitted
from the third reflector from a surface 211 that is opposite to the
surface facing the VCSEL device.
[0052] In the devices shown in FIGS. 2a-2d, reference is made to a
top-emitting configuration where laser emission takes place from
the surface of the devices opposite to the substrate end.
Alternatively, an extended cavity VCSEL may also be configured for
a bottom emitting device, shown in FIGS. 3a-3d. In FIGS. 3a-3d
elements that are equivalent and provide same functionality in
corresponding top-emitting counter-part devices shown in FIGS.
2a-2d, are labeled with similar reference numerals and their
description will not be repeated. Referring now to FIG. 3, a
conventional prior art two terminal bottom emitting device is shown
in FIG. 3a and described in the U.S. patent application Ser. No.
13/541,906, filed on Jul. 5, 2012, that issued as the U.S. Pat. No.
8,675,706 to Seurin et al. on Mar. 18, 2014, and assigned to
Assignee of this application. Contents of the above mentioned
application is being incorporated by reference in its entirety.
[0053] In a bottom emitting device, the emission surface is located
on the substrate side of the device. Accordingly, the top and
bottom contacts are located on the substrate end and at the active
layer end, respectively. It should be noted that the bottom
emitting device is typically mounted with the substrate side up,
such that the light emission in the device is still in an upward
direction (with respect to the plane of the substrate in this
illustration).
[0054] Basic principle of an extended cavity bottom emitting device
is similar to the top-emitting counterpart shown in FIGS. 2b-2d. To
configure an extended cavity `bottom emitting` VCSEL device shown
in FIGS. 3b-3d, the reflector 303 is made with a lower reflectivity
and placed between the reflector 306 and the third reflector 310,
respectively. Reflectivity and position of the third reflector with
respect to the reflector 303 is determined such that the phase
matched reflectivity from the combination of reflectors 306, 303
and 310 provides lasing in the VCSEL cavity. More specifically, in
the embodiment shown in FIG. 3b, the third reflector 310 is
integrated with the substrate 301. In the alternative embodiment
shown in FIG. 3c, the third reflector 310 is constructed on a
transparent substrate 313 and the transparent substrate is bonded
to the bottom emitting VCSEL device substantially in a manner
described in reference with FIG. 2c to construct a monolithic
combination. That description will not be repeated. In the
embodiment shown in FIG. 3d, the third reflector 310 is placed
external to the VCSEL device.
[0055] It will be apparent to those skilled in the art that in the
embodiments shown in FIGS. 3b-3d, the laser emission is through a
window 308 in the VCSEL substrate 301. Although devices in FIGS.
2a-2d and 3a-3d are shown with the substrate 201 and 301,
respectively, it is a common practice to reduce the thickness of
the substrate, or completely remove the substrate in some
applications, for efficient heat dissipation. One advantage of the
devices described in reference with FIGS. 2 and 3 is that the VCSEL
devices have planar contacts that do not use an external wire
bonding. As a result whole assembly can be constructed at the wafer
level to produce a multitude of modules on the wafer and then the
completed modules are separated out by dicing without having to
connect individual devices externally to individual contact
pads.
[0056] The VCSEL devices described here may advantageously be
extended to construct VCSEL arrays and arrays of VCSEL arrays, to
configure optical pump modules in a modular fashion as has been
described for conventional two terminal VCSEL devices in the U.S.
patent application Ser. No. 13/541,906, filed on Jul. 5, 2012, that
issued as the U.S. Pat. No. 8,675,706 to Seurin et al. on Mar. 18,
2014, and assigned to Assignee of this application. Accordingly,
three reflector extended cavity VCSEL devices may be configured in
an array as shown in FIG. 4. More specifically, FIG. 4a shows a two
dimensional VCSEL array 411 constructed from a plurality of VCSEL
devices (each dot 410 represents a VCSEL device) on a common
substrate 412. VCSEL arrays may be constructed from any type of
VCSEL device described in reference with FIGS. 2a-2c and 3a-3c.
[0057] All the VCSEL devices in the array are electrically
connected to the substrate which functions as a first common
terminal of the array. In order for the VCSELs to emit
collectively, the second electrical contact of each VCSEL in the
array is connected using a common metallization on the array
surface which functions as a second common terminal of the array.
One important aspect of the planar arrays constructed according to
this invention is that the contacts to the array are entirely
planar thereby eliminating the need for external wire bonding.
Those skilled in the art will be able to appreciate that
reliability of these VCSEL arrays is substantially higher than
conventional devices having external wire bonding.
[0058] For higher output power, all the VCSEL devices in the array
are configured to emit collectively in-phase, in the same direction
as shown by an upward arrow 414 in this example. For the ease of
description, the VCSEL array 412 as shown will be referred as VCSEL
array chip (or array chip hereinafter). In this particular example,
the array of VCSEL devices is arranged in a circular pattern. It
can be appreciated that array chips may be configured in any
regular geometric pattern or random shape. The array chip 412 can
be mounted on a thermal submount 413 that is described in the
co-authored and co-owned pending U.S. patent application Ser. No.
13/337,098 by Seurin et al., filed on Dec. 24, 2011. Content of the
above mentioned application is being incorporated by reference in
its entirety.
[0059] Referring now to FIG. 4b, there it shows a VCSEL array
configured with an external reflector. More specifically, an array
of multiple VCSEL devices constructed on a common substrate is
configured into a three reflector VCSEL array. A separate external
reflector 420 (equivalent of the third reflector in FIGS. 2d and
3d) is located at a pre-determined distance above the top of the
VCSEL array substrate 412 such that the combined cavity comprising
the three reflectors produces laser action 419 in each VCSEL device
with the desired characteristics. The output beam from the array of
VCSELs 414 is transmitted out in the upward direction (in this
particular example) from the external reflector 420.
[0060] It is noted that external cavity VCSEL array may be
constructed from top or bottom emitting VCSEL devices shown in
FIGS. 2a-2d or 3a-3d by appropriately reducing reflectivity in the
one of the reflector (206 or 306 or 203 and 303, respectively) as
the case may be. The entire module may be assembled in a wafer form
and a single third reflector may be bonded to the substrate with
the VCSEL array. In a variant embodiment, many modules may thus be
fabricated together and diced with the external reflector as well.
Those skilled in the art will be able to practice the invention in
many different variant fabrication processes that are well known in
the art.
[0061] As a matter of convention and not by way of limitation, in
the following discussion the term VCSEL arrays or VCSEL array
devices or elements will include any of the VCSEL devices including
VCSELs, extended cavity VCSELs and external cavity VCSELs or array
chips of VCSELs, described in reference with FIGS. 2a-2d, 3a-3d and
4a-4b. Using these VCSEL devices, optical modules may be
constructed. It is known that emission from single VCSEL is narrow
and may be coupled to an optical fiber with relative ease. However,
to obtain high power from VCSEL module, as will be necessary to
configure a high output power VCSEL pump module for a fiber gain
system, additional optical elements may be necessary to facilitate
beam shaping from each VCSEL in the array or of the entire array as
the case may be, depending upon the application.
[0062] Referring now to an embodiment shown in FIG. 5, one option
for beam shaping is to use an array of microlens to collimate
output beams from individual VCSELs in the array device. More
specifically, a VCSEL array device 510 and the microlens array lens
elements 517 are designed and fabricated so that the individual
devices and elements are aligned on the same axes. The VCSEL array
substrate 512 is mounted on a heat sink 513 for effective cooling
of VCSEL devices. The microlens array is aligned and positioned at
a pre-determined distance from the VCSEL array so that the
diverging beams 515 from the VCSEL elements are formed into an
array of collimated beams 516 with the same array pitch and form
factor as the VCSEL array and microlens array. The resulting
combined output beam that appears to have a quasi-uniform intensity
cross section with much lower divergence than the original output
beams from the VCSEL array.
[0063] In practice, other optical arrangements may be necessary to
efficiently couple output light from VCSEL array for pumping a
fiber and in particular a double-clad fiber frequently used for
fiber gain systems such as a fiber laser or amplifier. A few
exemplary arrangements that are particularly suited for VCSEL pump
modules are shown in FIG. 6. Referring now to FIG. 6a there it
shows an arrangement to directly couple light from a VCSEL array to
a double-clad optical fiber. However, these arrangements may also
be used for coupling pump light to a single-clad fiber.
Alternatively, pump light may be coupled to a single-clad fiber
which is then coupled to a double-clad fiber or a double-clad fiber
gain element, preferably by fusion splicing.
[0064] More specifically, a VCSEL array 612, having a plurality of
VCSEL devices collectively generate laser output light 614 in a
certain area which is coupled to a double-clad fiber 611 comprising
a fiber gain system. The double-clad fiber is well known in the art
and typically comprises a central core region 621 surrounded by an
inner cladding region 620 and an outer cladding region 622,
respectively. The arrangements shown and described here are only
exemplary and should not be construed to be the only options. Other
methods for coupling output of a VCSEL array to single or
double-clad fiber may also be used.
[0065] Refractive indices of the inner and outer cladding regions
in a double-clad fiber are selected such that the pump radiation is
confined between the inner and outer cladding regions to surround
the core region for efficient coupling of the pump radiation to the
core region. The core region 621 is typically doped with optically
active ions that need to be excited by the pump laser output 614 to
configure a fiber laser or a fiber amplifier (different from the
VCSEL arrays that form the source for optically pumping the fiber
laser). This arrangement is particular appropriate when the inner
cladding diameter is smaller or about the same size as the VCSEL
array emission area. The VCSEL array may or may not include a
microlens array.
[0066] FIG. 6(b) shows another arrangement for coupling light from
VCSEL pump laser to a fiber. In particular, pump radiation from a
VCSEL array 612 including a microlens array 615, uses an external
focusing lens 617 to couple the collimated output beams 616 into
the inner cladding region 620 of the double-clad fiber 611 at the
input of the fiber gain system (for example a fiber laser or
amplifier). The light in the inner cladding is confined in this
region by the lower index outer cladding 622. The input end of the
double-clad fiber 611 usually does not include a core region 621
over a short distance since the fiber laser or amplifier radiation
is normally confined to other regions of the fiber. However, it is
not unusual to have a core region at the input end of the
double-clad fiber.
[0067] In yet another arrangement shown in FIG. 6c an optical taper
end is used for coupling pump laser light to a double-clad fiber
depending on various circumstances that includes but are not
limited to, size of the array 612 and properties of the fiber 611.
More specifically, a taper optical component 623 with a high index
core and lower index cladding may be used instead of a focusing
lens. A collimated beam 616 from the VCSEL array pump laser 612 and
corresponding microlens array 615 is coupled to the fiber through
the taper section 623. The beams 624 are reflected from the tapered
sides of the core region of the component into the fiber inner
cladding 620. It should be noted that light from the VCSEL array
may also be coupled to a taper end without the microlens array. The
choice of coupling method is determined by the output power to be
coupled and the application. While only few examples are being
described here, many other combinations and sub-combinations of
these methods may be apparent to those skilled in the art.
[0068] One aspect of this invention is to provide a high optical
power pump module for pumping a fiber gain system, for example, a
fiber laser or a fiber amplifier. VCSEL devices and in particular,
extended and external cavity VCSEL devices described in reference
with FIGS. 2 and 3 generate high power output in a single mode
thereby resulting in very high quality output beam. VCSEL arrays
constructed from these types of VCSEL devices are ideally suited to
construct very high power optical pump module in a small foot
print. Typical output optical power obtained from these modules may
be in the range of 20-200 W in CW operation and 200 W-2 kW (kilo
Watt) in pulsed operation. It is important to note that to obtain
similar power output from conventional optical pumps known in the
art it would require very elaborate cooling systems to cool optical
pumps. Pump module constructed from VCSEL arrays has a unique
advantage in this regard and has been disclosed in the U.S. patent
application Ser. No. 13/541,906 filed on Jul. 5, 2012, that issued
as the U.S. Pat. No. 8,675,706 to Seurin et al. on Mar. 18, 2014,
and assigned to Assignee of this application, contents of which, is
being incorporated by reference in its entirety.
[0069] Referring now to FIG. 7, there it shows one embodiment of an
optical pump module constructed from VCSEL arrays. More
specifically, an optical pump module 701 comprises a module housing
having two parts; a housing baseplate 730 and a cover 731. The
housing cover further includes a fiber alignment guide 728 and a
bend radius limiter 729, respectively, to securely hold an optical
fiber pigtail 719. It should be noted that the pump module shown
here is merely exemplary and other physical shapes and dimensions
may be employed depending upon the output power and heat
dissipation requirement of the pump module. The pump module further
includes an optical source comprising a VCSEL array 712 placed on
submount 713 for example, a printed circuit high thermal
conductivity ceramic or other similar platform suitable for
constructing planar contacts. The submount is bonded to a high
thermal conductivity base 723.
[0070] A fiber assembly mount 727 holding an alignment housing 726
is included to hold the fiber pigtail 719 to be aligned with the
optical source. The fiber pigtail is fed through the alignment
guide 728 and the bend radius limiter 729. And while other types of
fiber pigtail may be used, a double-clad fiber pigtail is preferred
in this embodiment for constructing high optical power pump module,
particularly for use with a fiber laser or fiber amplifier having a
gain element comprising a double-clad fiber. In order to obtain a
collimated pump beam a microlens array 715 is supported on an
independent mount 724. The microlens array is disposed at a
pre-determined distance from the VCSEL array and the two are
aligned so as to generate a collimated pump beam having a uniform
intensity as has been described in reference with FIG. 5.
Alternatively, the microlens array may be integrated with the VCSEL
array at a wafer processing level.
[0071] In this exemplary embodiment the collimated pump radiation
from the VCSEL array is coupled to the fiber pigtail using a
focusing lens 716. The focusing lens is supported on a second
independent mount 725. It should be noted that other coupling
arrangements described earlier in reference with FIG. 6 for
example, direct coupling or using a tapered waveguide will be
equally effective. The focusing lens is positioned at a distance
from the microlens array to focus the collimated pump beam on to
the fiber pig tail. It should be noted that the VCSEL array, the
microlens array, the focusing lens and the fiber alignment housing
may be maneuvered independently for alignment.
[0072] Once all the components are aligned, they are bonded to the
module housing baseplate 730. The fiber pigtail is sealed with the
alignment guide and the bend radius limiter. The housing cover and
baseplate may be environmentally sealed. The baseplate 730 may be
bonded to a heat dissipation device for example, a heat sink for
cooling the VCSEL array through the thermal mount 723. Cooling may
be facilitated by a circulating fluid, for example, air, a gas or a
liquid coolant through the heat sink, as has been described in the
U.S. patent application Ser. No. 13/541,906 filed on Jul. 5, 2012,
that issued as the U.S. Pat. No. 8,675,706 to Seurin et al. on Mar.
18, 2014, and assigned to Assignee of this application.
Fiber Laser:
[0073] Referring now simultaneously to FIGS. 7 and 1, the VCSEL
pump module 701 described in the previous section may be used to
configure a fiber gain system 100 described in reference with FIG.
1. More specifically, the optical pump module 701 replaces the
block referred as pump module 101 in FIG. 1. In an exemplary
embodiment, a fiber laser shown in FIG. 8 is configured according
to the concepts outlined in the previous sections. The fiber laser
is constructed using a gain element 842 comprising a single or
double-clad fiber section for example. The double-clad fiber (shown
in the inset) selected for this embodiment includes a core region
843 doped with active ions that generate the lasing action. The
core region is surrounded by an inner cladding region 844 and an
outer cladding region 845, respectively. It should be noted that
other types of gain elements may be pumped equally well using a
VCSEL pump module.
[0074] The inner cladding region is transparent to the pump
radiation. The refractive indices of the core, inner cladding and
outer cladding regions are selected such that the pump radiation is
confined within the inner and outer cladding regions. The
wavelength of the pump radiation is selected to match the
absorption band of the ions doped in the core of the gain element
fiber for effective absorption of the pump radiation. The pump
radiation propagates in the inner cladding region at various angles
to the fiber axis so that the pump radiation also propagates across
and through the core region. The ions in the core region absorb the
pump radiation and transition to an excited state having a higher
energy. Upon returning to the ground state, the excess energy
manifests as amplified light. The doping ions can be one or more of
various types of ions and correspond to the particular operating
wavelength desired from the fiber laser.
[0075] The gain element is placed between reflectors 840 and 846 to
construct a fiber laser resonant cavity. The reflectors are
typically fiber Bragg gratings which are effective in the core
region but have no effect on the pump radiation in the inner
cladding region. In this example the reflector 840 has high
reflectivity whereas the second reflector 846 having relatively low
reflectivity is configured to be the laser output port. The gain
element may be spliced to the reflectors or connected using low
loss connectors 833 as shown in FIG. 8. In practice, input and
output fiber sections 834 and 836, respectively are spliced to the
reflector ends of the gain element to connect it to a pump module
or to an output port. Alternatively, low loss connector may also be
used.
[0076] A fiber pigtail 819 from a pump module 801 is spliced
through the gain element via the input fiber 834 at the reflector
840. The output of the fiber laser is emitted through the output
fiber section 836 as a diverging beam. An additional lens 838 may
be used for obtaining a collimated output beam 839. An optional
isolator 848 may be disposed between the output reflector 846 and
the output fiber section 836 for preventing reflected light from
objects illuminated by the output beam, into the fiber laser which
may result in destabilizing the fiber laser, particularly when the
internal gain in the gain element is high. An additional option is
to coat the fiber end appropriately with antireflection coatings
-and/or angle the fiber to reduce reflected light from the fiber
end destabilizing the laser.
[0077] In one variant embodiment of the invention, particularly
when internal gain in the gain element is relatively high,
reflectivity of the reflector at the output end (846 in this
example) for optimum fiber laser operation needs to be quite low.
If the required reflection coefficient required is approximately 4%
then instead of a reflector 846 (for example, a fiber Bragg
grating) a perpendicular cleaved end of a fiber is preferred. In
one embodiment of the invention shown in FIG. 9, a perpendicular
cleaved end at the output of the laser is advantageously used for
optically pumping the gain element from both ends (dual pumping
configuration), particularly when a high pump power is required to
obtain high power laser output.
[0078] More specifically, a gain element 935 in this example
comprises a double-clad fiber similar to the one described in
reference with FIG. 8. One end of the gain element is connected or
preferably spliced, to a reflector for example, a fiber Bragg
grating 940 and the other end 936 is cleaved perpendicular to the
lasing axis. A cleaved fiber end functions as a low reflectivity
reflector and forms the feedback cavity with the reflector on other
end of the gain element. An optical pump module 901 comprising a
VCSEL array is connected to the other end of the reflector using a
fiber splice or a low loss connector 933 (in this example). VCSEL
optical pumping at this end is substantially similar to the one
described in reference with FIG. 8 and will not be repeated for
brevity.
[0079] The perpendicular cleaved end 936 of the fiber also provides
laser output 921 which is collimated by one or more optical
elements for example, a lens 922 in this case to provide a
collimated laser output 909. This arrangement is particularly
suited for applying more input pump radiation from the second end
of the gain element 935. More specifically, additional pump
radiation from one or more VCSEL arrays 925 (only one labeled for
clarity) each having a respective microlens array, may be arranged
on a thermal mount 926 around the fiber laser output beam. The
thermal mount 926 is constructed from a thermally conducting
material to provide cooling for the VCSEL arrays. Pump radiation
927 from the VCSEL arrays and microlens assembly is coupled into
the inner cladding of the output end 936 (dual pumping) of the
fiber laser using the lens 922. A central hole in the aperture
mount allows transmission of the output beam 909 from the core of
the output fiber 936 through the collimating lens 922.
[0080] Referring back to the fiber laser configuration shown in
FIG. 8, the fiber laser as described therein may be operated in
different modes including Continuous Wave (CW), Quasi-Continuous
Wave (QCW) or Pulse (P) modes. In practice, a pump module and in
particular a VCSEL array pump module may be operated by applying
current continuously for CW operation or by using a pulsed current
source at a pre-determined duty cycle for QCW or pulse operation,
depending upon the required pulse width at the output of the fiber
laser. In one embodiment a pulse generator may be employed to
generate an electrical pulse with the pulse length and repetition
rate required for the output pulse width for the fiber laser
optical output. The output from the pulse generator is amplified to
generate a high current drive pulse to drive the VCSEL array pump
module. The high energy pumping pulse transmitted through the inner
cladding of the pigtail fiber 819 is coupled to the inner cladding
region of the gain element and amplifies the fiber laser beam
propagating in the core approximately through the duration of the
VCSEL pump pulse.
[0081] The pulsed fiber laser configuration described above in
reference with FIG. 8 is particularly suitable for generating
medium length pulses (1-100 microsecond). Generation of shorter
pulse in the fiber laser is primarily limited by the dynamics and
in particular, the excited state lifetime of the ions that generate
the lasing action in the core (843) of the fiber gain element.
Therefore, for generating very short laser pulses (1-100
nanosecond), especially in laser systems with gain element with
long excited state lifetime well known method of Q-switching is
more suitable. FIG. 10 shows an embodiment that may be used to
configure a Q-switched fiber laser according to this invention.
Principle of Q-switching is well known in the art and will not be
elaborated further.
[0082] More specifically, the embodiment shown in FIG. 10 comprises
a basic fiber laser layout similar to the one described in
reference with FIG. 8. Elements that have similar reference
numerals are equivalent or provide substantially similar
functionality and that description will not be repeated. In this
configuration the VCSEL pump module 1001 is operated in pulse mode
using a function generator 1054 to generate current driving pulse
1061, the length of which is long enough for significant excitation
of the gain fiber 1042. The output pulse 1061 from the function
generator is further amplified in a current driver 1055 for driving
the VCSEL array(s) of the optical pump module. The pump pulse is
transmitted to the inner cladding layer of the pigtail 1019 which
is connected to the input fiber section 1034 of the gain element
1042. The gain element is followed by a Q-switch element 1056 which
includes but is not limited to, a saturable absorber fiber
component, a fiber coupled acousto-optic switch, a SBS (Stimulated
Brillouin Scattering) non-linear component and many other devices
that are well known in the art for Q-switching and may be selected
on their independent merits depending upon the application.
[0083] In operation, the ions in the core region of the gain
element absorb the pump radiation and acquire a high population
density in higher excited states for a period determined by the
excited state lifetime. The ions in the excited state have very
high internal gain, thereby building a high power pulse very
rapidly. When the Q-switch 1056 is activated laser action rapidly
builds up and a very short high energy fiber laser pulse 1062 is
transmitted to the output end of the fiber through the output
reflector 1046, isolator 1048, the output fiber and the collimating
lens 1036, respectively. The fiber laser output 1009 in this
instance is a succession of high energy short pulses. The
repetition rate of this sequence is determined by the rate at which
the q-switch is activated. As the high energy pulse is transmitted
out of the gain element, the ions in the core region return to the
ground state. As a result, the gain build up in the core region
terminates rapidly until a new electrical pulse is provided to the
VCSEL array to provide optical pumping to the fiber laser.
[0084] One advantage of the fiber laser configured according to
this invention is that most of the VCSEL array pump power input to
the fiber laser gain element is effectively utilized in generating
high output power from the fiber laser. The VCSEL pump radiation is
emitted in a narrow wavelength band which is significantly narrower
than an edge emitter semiconductor laser. By appropriately
selecting the VCSEL emission wavelength, it is very well matched to
the absorption band of the gain ions so that a high percentage of
the pump radiation is absorbed by the gain ions. In FIG. 11, output
power from a fiber laser similar to one described in reference with
FIG. 8 (or 10), and remaining pump power not absorbed by the fiber
laser are plotted as a function of pump power (x-axis) are shown in
traces 1101 (left y-axis) and 1102 (right y-axis), respectively.
The amount of pump radiation which is not absorbed by the gain
fiber is separated from the fiber laser beam by a dichroic
reflector or an optical filter.
[0085] From the graph shown in FIG. 11, fiber laser output power as
a function of absorbed pump power is plotted as shown in FIG. 12.
The graph shown in this plot indicates that the fiber laser output
power is linear with the absorbed pump power. The inset shows the
optical spectrum of the fiber laser output radiation. A detail
output spectrum of the fiber laser output is shown in FIG. 13 in
linear scale (a) and logarithmic scales (b), respectively. More
specifically, FIG. 12a shows a normalized plot of the output
spectrum whereas, FIG. 12b shows the intensity plotted (y-axis) in
dBm as a function of wavelength. The spectral linewidth is about
0.27 nm for this exemplary fiber laser.
Fiber Amplifier:
[0086] Referring back to FIG. 1, a fiber gain system shown therein
may be configured as a fiber amplifier. One embodiment of a fiber
amplifier configured using VCSEL array optical pump module is shown
in FIG. 14. A fiber amplifier has many components that are similar
to the fiber laser described in reference with FIG. 8. More
specifically, a fiber amplifier comprises a gain element 1442
including a core region, an inner cladding region and an outer
cladding region 1443, 1444 and 1445, respectively (shown in the
inset) that have substantially the same functionality described
earlier in reference with FIG. 8. The core region includes active
ions that generates optical gain and contributes to the
amplification of an optical signal. While the principles are being
described using this particular configuration, same principle may
be extended to configure other types of fiber amplifier.
[0087] Pump radiation is generated in a pump module 1401 that
comprise a VCSEL array to couple light to the inner cladding of an
input port whereas the optical signal 1440 to be amplified is
connected to the core of the input port of an optical combiner
1447. The output port of the combiner comprises a double-clad fiber
1434 that matches the cross-section of the fiber gain element input
section 1441. In effect, the input signal propagates in the core
region and the pump radiation propagates in the inner cladding and
core regions of the combiner and fiber gain elements. An optical
isolator 1448 is placed between the fiber gain element 1442 and the
input fiber section 1441 to control the light only in the core.
More specifically, the isolator facilitates transmission of the
input signal to be amplified into the fiber gain element input in
the forward direction but prevents reflection of radiation from the
gain element traveling in the backwards direction going back into
the fiber amplifier gain element to prevent destabilization of the
fiber amplifier.
[0088] Amplified signal is transmitted out of the gain element
through a second isolator 1448 placed on the right side of the gain
element in this example. The isolator prevents any reflected signal
from being transmitted back into the gain fiber and destabilizing
the fiber amplifier. The second isolator is connected to an output
fiber section 1436 preferably by fusion splicing, or via a low loss
coupler 1433. Typically, the output from the amplifier is a
diverging beam which is collimated using additional optical
elements for example, a simple lens 1438 in this exemplary case,
into an amplified output signal 1409. Other ways of beam shaping
may also be used depending upon the application and these methods
are well known in the art.
[0089] Furthermore, to achieve higher gain and output power from
the fiber amplifier, multiple stages of amplifiers may be
constructed by cascading, namely the output 1409 in output fiber
1436 from one stage is connected to an input coupler similar to
1447 together with a second optical pump module similar to 1401 to
construct a second stage amplifier. It may further be recognized
that each stage of amplification in a multi-stage configuration may
be substantially similar or may differ depending upon the level of
amplification required at each stage. For example, each successive
amplifier stage may be designed to handle a different level of
input and output optical power. It is also important to ascertain
that the physical dimension of the double-clad fiber gain element
is matched well with the double-clad fiber used for input and
output port pigtails.
[0090] In a further variation, an output from a fiber laser similar
to the one described in reference with FIG. 8 may be appropriately
combined with a fiber amplifier stage similar to the one described
in reference with FIG. 14 to configure a CW or pulse mode fiber
laser with high output power. In this configuration, a first stage
of a gain element is configured as a laser to be operated in CW,
QCW or pulse mode. Non-collimated output from the first stage is
appropriately combined to a second stage of gain element configured
as a fiber amplifier. These and other combinations and
sub-combinations that may occur to those practicing the art are
included within the broad framework of the description provided
here.
[0091] It may be appreciated by those skilled in the art that the
input signal 1440 to be amplified may be any type of CW, QCW, pulse
including very short pulse, or multiplexed signal such as a
wavelength division multiplexed (WDM) signal. Advantageously, a
fiber amplifier configured as described may provide amplification
for optical signal having specialized characteristic for example,
very short pulses, very low noise CW optical signals, very narrow
linewidth signals and FM modulated signals for Doppler LIDAR
applications, just to name a few. The fiber amplifier using pump
modules including VCSEL arrays provide an ideal solution for
amplifying these types of optical signals where preserving high
quality and special characteristics of the signal is important.
Regenerative Fiber Laser:
[0092] In one embodiment shown in FIG. 15, a VCSEL pumped
regenerative fiber laser is provided by combining a fiber laser
stage with a fiber amplifier stage. The configuration also known as
Master Oscillator Pulse Amplifier (MOPA) is particularly useful in
preserving pulse quality of a short laser pulse while amplifying
the pulse to provide very high output power, for example. In
particular, a fiber gain element 1542 including a core, an inner
cladding and an outer cladding region, is set up in regenerative
loop configuration. The regenerative loop includes an acousto-optic
switch 1560 placed with the gain element between two reflectors
1540 and 1550, respectively, to provide a feedback resonator. The
reflectors may include but is not limited to fiber Bragg gratings
as shown in this exemplary embodiment. The acousto-optic switch
shown in this example has three ports 1561, 1562 and 1563,
respectively. In a closed position of the acousto-optic switch
light entering the core region of the fiber gain element traverses
between ports 1561 and 1562 to the high reflectivity fiber Bragg
grating. In an open position of the acousto-optic switch, light
entering at port 1561 is transmitted to port 1563 and port 1562 is
blocked.
[0093] A pump module 1501 comprising VCSEL arrays is connected to
one of the reflectors 1550 to pump the gain element 1542. For this
application the pump module is operated in pulse mode. The pump
radiation is coupled to the inner cladding region of the fiber gain
element through the inner cladding region of the fiber Bragg
grating 1560. The pump radiation pulse is relatively long so that a
large proportion of the ions in the core of the fiber gain element
absorb the pump radiation and transition to the excited state for a
period of time governed by the upper excited state lifetime.
[0094] A pulse source 1510 generates pulses to be amplified which
for the purpose of discussion will be referred as a `seed` laser
pulse. The seed laser pulses may be generated by another laser or
other types of optical sources producing short pulses with the
required wavelength and pulse length characteristics. The seed
laser pulses enter the regenerative loop via a circulator 1570 at
an input port (or input fiber) 1571 and couples to the
acousto-optic switch at a port 1563 through a second port 1572 of
the circulator.
[0095] In operation, at a specified time synchronized with the
VCSEL pump module pulse, when the acousto-optic switch is in `open`
position, a seed laser pulse from the pulse source is transmitted
to the core of the fiber gain element via port 1561. Immediately
after the seed laser pulse passes through the switch the
acousto-optic switch is closed. The seed laser pulse transmits
through the core of the fiber gain element and is amplified by the
excited ions in the core region of the gain element. The amplified
seed laser pulse is reflected by the fiber Bragg grating 1550 back
into the core region of the fiber gain element and amplified
further. The amplified seed laser pulse is transmitted by the now
closed acousto-optic switch from port 1561 to port 1562 and to the
other fiber Bragg grating reflector 1540.
[0096] As a consequence, in each pass the seed pulse is thus
reflected back and forth through the fiber gain element and is
amplified continuously. At a suitable time when the seed laser
pulse has depleted the fiber gain element and reached its maximum
energy, the acousto-optic switch is opened at the point such that
the amplified high energy seed laser pulse may be transmitted from
port 1561 to port 1563 into the core of fiber connecting the second
circulator port 1572 to the acousto-optic switch port 1563 which
enables the amplified seed laser pulse to exit from the circulator
port 1573 as the output pulse 1509.
[0097] One advantage of the fiber gain systems designed according
to this invention is that the VCSEL pump power may be increased
further by passively coupling pump radiation from several pump
modules using star couplers for example, as is well known in the
art. However, a star coupler configured according to this invention
requires double-clad fiber for matching the fiber employed in the
gain element of a fiber laser or amplifier. Additional pump modules
may be employed in a modular fashion. In general, for efficient
coupling of radiation from one fiber to the other, while connecting
double-cladding fibers either by fusion splicing or by low loss
connector, it is important that the physical dimension of the core
and cladding regions, particularly the inner cladding region is
well matched. The star couples may be placed before or after the
reflectors or isolators as the case may be.
[0098] As shown in FIG. 16, one embodiment of exemplary star
couplers comprising double-clad fiber is provided. This
configuration may be used to generate higher pump power for fiber
laser or for an amplifier (with some slight modification) and is
particularly suitable for pump modules comprising VCSEL emitter
arrays. Due the small form factor of VCSEL array modules, high pump
power modules may be configured in a relatively small physical
area. The basic design principle for high power start coupler using
double-clad fiber is similar for both the applications. High power
star couplers may be configured in single end pumping as well as
dual end pumping configuration. In particular, pump beam from
several pump modules 1601 are combined in an optical coupler 1650
placed at the input end of the gain element 1635. The fiber
combiner is designed to couple pump beams from respective inner
cladding of each input fiber 1619 (only one labeled) into the inner
cladding of the output fiber 1634 of the coupler.
[0099] In case of a fiber laser configuration shown in FIG. 16, the
coupler may be placed before the reflector 1640 and connected to
the coupler at a designated port such that only the laser signal
propagates through the core region of that port. The output of the
coupler is designed to have substantially similar dimensions as the
gain element double-clad fiber. The combined pump beam form
multiple VCSEL pump modules is transmitted directly to the inner
cladding of the gain fiber, whereas the laser emission is confined
in the core region of the gain element. In an alternative
arrangement, the fiber reflector may be placed after the coupler
and the output port of the fiber reflector is matched with the gain
element. Furthermore, in a configuration where the fiber laser is
pumped only from one end, the other end of the gain element is
directly connected to the second reflector 1646.
[0100] However, for higher output power, the laser may be optically
pumped from the other end of the gain element also shown in FIG.
16. A second coupler 1651 substantially similar to the coupler 1650
is connected between the gain element and the second reflector. One
or more optical pump modules are connected to this end of the gain
element as well. The coupler is designed to have a fiber dedicated
to connect the reflector and the output fiber 1636 from the
reflector propagates the laser output. In both arrangements, one of
the combiner 1651 input fibers has a core region which is directly
connected to the core of the coupler output fiber 1636. A fiber
collimator 1652 including a fiber and lens may optionally be used
to collimate the diverging beam into a free space parallel beam by
a lens.
[0101] The arrangement shown in FIG. 16 with a little modification
is also applicable for adding VCSEL pump modules for fiber
amplifier application. In this embodiment, the fiber reflectors
1640 and 1646 are replaced by isolators. Similar to the fiber laser
configuration, the isolators may be placed before or after the
couplers 1650 and 1651 and the coupler ports may accordingly be
designed to match the gain element appropriately. Those skilled in
the art may appreciate that the pump power may be increased in a
modular fashion with relative ease and additional cost as compared
to conventional optical pumps, even those including single VCSEL
devices or linear arrays of VCSEL devices.
[0102] Although the invention has been described with particular
reference to preferred embodiments of VCSEL and VCSEL array pump
modules to configure fiber gain systems for different applications,
these specific examples are merely to illustrate systems where the
high power pump modules according to this invention may be used.
Pump modules described in this invention may be used in other fiber
gain systems. Pump modules described here show regular two
dimensional VCSEL arrays of a specified shape. However, single high
power VCSELs, linear arrays or two dimensional arrays including
arrays of different shapes, or arrays of one or two dimensional
arrays of VCSELs may be used to configure pump modules emitting in
different shapes depending on the requirements of the particular
fiber laser pumping application. Variations and modifications of
these combinations, and sub-combinations that will be apparent to
those skilled in the art are included within the broad framework of
the invention that is captured in the appended claims.
* * * * *